Magnetic random access memory (MRAM) cells are often based on a magnetic tunnel junction (MTJ) cell comprising at least three basic layers: a “free” ferromagnetic layer, an insulating tunneling barrier, and a “pinned” ferromagnetic layer. In the free layer, magnetization moments are free to rotate under an external magnetic field, but the magnetic moments in the “pinned” layer are not. The pinned layer may comprise a ferromagnetic material and/or an anti-ferromagnetic material which “pins” the magnetic moments in the ferromagnetic layer. A thin insulation layer forms a tunneling barrier between the pinned and free magnetic layers.
In order to sense states in the MTJ configuration, a constant current can be applied through the cell. As the magneto-resistance varies according to the state stored in the cell, the voltage over the memory cell can be sensed. To write or change the state in the memory cell, an external magnetic field can be applied that is sufficient to completely switch the direction of the magnetic moments of the free magnetic layer.
A Tunneling Magneto-Resistance (TMR) effect is often utilized in conventional MTJ configurations. The TMR effect allows magnetic moments to switch directions in a magnetic layer in response to exposure to an external magnetic field. By utilizing the TMR effect, the magneto-resistance (MR) of an MTJ configuration may be altered. MR is a measure of the ease with which electrons may flow through the free layer, the tunneling barrier, and the pinned layer. A minimum MR occurs in an MTJ configuration when the magnetic moments in both magnetic layers have the same direction or are “parallel.” A maximum MR occurs when the magnetic moments of both magnetic layers are in opposite directions or are “anti-parallel.”
MRAM cells typically include a write line that is perpendicular to the hard (short) axis of the free layer. An electrical current is introduced to the write line to create the magnetic field necessary to change the direction of the free layer magnetization. However, the parallel alignment of the write line and hard axis of the free layer requires one of the highest amplitudes of magnetization (Hc) to change the direction of the free layer magnetization when considering all possible orientations of the write line relative to the hard axis of the free layer. Consequently, such an orientation also requires the highest write current levels.